The present invention relates generally to a device for illuminating a display, and, more particularly, to an illumination device which uniformly injects light into an optical cavity, such as a waveguide.
Illumination devices are used to illuminate non-emissive displays, such as LCD displays for personal digital assistants or cellular phones. One type of illumination device comprises a waveguide having a plurality of light sources disposed along the periphery of the waveguide. The light sources are energized and the emitted light illuminates the waveguide.
It is highly desirable for the illumination to be bright and substantially uniform across the area of the display. However, in prior art, the intensity of the light from the light sources tends to decrease as the distance from the light source increases, thereby causing undesirable bright spots along the periphery of the display where the light sources are mounted.
Miniature fluorescent tubes are commonly used as light sources. Such tubes do not produce much heat, and thus, minimize heat build-up. However, they are not very bright. Solid state point light sources, such as LEDs, are relatively bright, but have relatively small thermal mass which causes a relatively high amount of heat build-up. Such heat build-up can cause the level of light output of the LEDs to significantly decrease over time after the LED is initially energized. Consequently, the brightness of the illumination is degraded.
In view of the foregoing, there is a need for a compact device and method for injecting light into the periphery of a waveguide such that the waveguide provides a bright, uniform illumination for the display.